The present invention relates to a semiconductor device module used in a range finder module mounted an automatic focusing camera. Specifically, the present invention relates to the structure of a sensor stage that mounts a semiconductor optical sensor chip thereon.
FIG. 3 is an exploded perspective view showing the module structure of the related range finder disclosed by the present inventors in Japanese patent application No. 2001-328568, on which U.S. patent application Ser. No. 10/004,269 is based. Referring now to FIG. 3, the related range finder module includes an optical lens mount 1 including a pair of lenses 1L and 1R arranged side by side, an aperture mount 2 including a pair of aperture holes 2L and 2R corresponding to the lenses 1L and 1R, and a sensor stage 3 including a semiconductor optical sensor chip 4 mounted thereon. Hereinafter, the semiconductor optical sensor chip will be referred to simply as the “sensor chip”. The sensor stage 3 is a package of a semiconductor device module housing the sensor chip 4. The optical lens mount 1, the aperture mount 2 and the sensor stage 3 are made of plastic. These constituent elements are piled up and bonded at the bonding planes thereof such that these constituent elements are assembled into a range finder module, the inside space thereof is filled with a transparent filler gel.
The sensor chip 4 is a rectangular plate, on which photo-sensor arrays 4L and 4R are aligned in the longitudinal direction of the sensor chip 4. Lead terminals (dual-in-lines) 5 are formed by insertion molding into the sensor stage 3 made of a resin. The sensor chip 4 is fixed with an adhesive at a predetermined location on a bottom wall 3a of the sensor stage 3. Electrode sections 4a (hereinafter referred to as “bonding pads”) arranged along both sides of the central surface portion of the sensor chip 4 and inner lead sections 5a of the lead terminals 5 led out onto the bottom wall 3a of the sensor stage 3 are connected to each other via bonding wires 6. Windows formed on both sides of the bottom wall 3a of the sensor stage 3 function as injection ports, from which a transparent filler is injected into the range finder module when assembled.
The related structure of the sensor stage 3 will now be described more in detail below with reference to FIGS. 4(a) through 5(b). FIG. 4(a) is a perspective view of the sensor stage 3. FIG. 4(b) is a top plan view of the sensor stage 3. FIG. 5(a) is a top plan view of the sensor stage 3 with a sensor chip 4 mounted thereon. FIG. 5(b) is a cross section along A—A of FIG. 5(a). Referring now to these figures, a channel groove 3b, the lateral cross section thereof is shaped with a letter U, is formed in the central surface portion of the bottom wall 3a of the sensor stage 3. Hereinafter, the channel groove 3b will be referred to as the “U-groove”. The width of the U-groove 3b is set corresponding to the width of the sensor chip 4. The sensor stage 3 includes a pair of pedestals 3c formed in the U-groove 3b along the side walls thereof, and pins 3d protruding from the both open end bottom portions of the U-groove 3b. The sensor chip 4 is mounted on the pedestals 3c. 
The pedestals 3c work as receptacles when the bonding wires are connected to the bonding pads 4a on the sensor chip 4. The pedestals 3c facing to each other across the U-groove facilitate retaining the adhesive in the central potion of the U-groove. As shown in FIG. 5(a), the width of the pedestals 3c is set corresponding to the regions of the sensor chip 4, therein the bonding pads 4a are arranged. The pins 3d are disposed to prevent the sensor chip 4 mounted on the sensor stage 3 from slanting in the longitudinal direction thereof.
The sensor chip 4 is mounted on the sensor stage 3 in the following way. A thermosetting and UV-curing adhesive is coated on the U-groove 3b formed in the bottom wall 3a of a sensor stage 3. Then, a sensor chip 4 is pressed onto a predetermined location in the U-groove 3b. An ultraviolet ray is irradiated onto the sensor stage 3 with the sensor chip 4 put thereon to temporarily fix the sensor chip 4 at a predetermined location on the sensor stage 3. Then, the sensor chip 4 is finally fixed at the predetermined location on the sensor stage 3 by thermally setting the adhesive. The bonding wires 6 are connected between the inner lead sections 5a of the lead terminals 5 formed by insertion molding into the sensor stage 3 and the bonding pads 4a on the sensor chip 4.
The thermosetting and UV-curing adhesive is used to bond the sensor chip 4 accurately at the predetermined location on the sensor stage 3 efficiently and reliably. The sensor chip 4 is fixed accurately at the predetermined location on the sensor stage 3 temporarily by the UV-curing of the adhesive and tight adhesion is secured by the thermosetting. As described above with reference to FIGS. 4(a) through 5(b), the adhesive is provided with a certain thickness by securing a gap for retaining the adhesive in the central portion of the U-groove 3b by means of the pedestals 3c disposed along the respective side walls of the U-groove 3b and by filling the adhesive in the gap between the bottom of the U-groove 3b and the back surface of the sensor chip 4 mounted on the pedestals 3c so that displacement and parting of the sensor chip 4 caused by thermal expansion difference between the plastic sensor stage and the silicon sensor chip may be prevented from occurring. Thus, a very reliable range finder module is obtained.
The assembly process of mounting a sensor chip on the sensor stage having the related structure described above poses several problems. For example, in the step of coating an adhesive on the U-groove 3b formed in the bottom wall 3a of the sensor stage 3, the adhesive is coated also on the upper faces of the pedestals 3c formed stepwise along the side walls of the U-groove 3b. When a sensor chip 4 is pressed onto the sensor stage 3 in the subsequent mounting step, a part of the fluid adhesive coated on the U-groove 3b sometimes has no choice but to escape through the gaps left between U-groove 3b and the side edges of the sensor chip 4. The escaping adhesive splashes onto the upper faces of the bottom wall 3a and sticks to the inner lead sections 5a of the lead terminals 5 and the bonding pads 4a of the sensor chip 4. The amount of the adhesive in the central portion of the U-groove reduces due to the escape described above. The reduction in the adhesive amount causes voids between the sensor stage 4 and the bottom of the U-groove, leaving some back surface areas of the sensor chip 4 not bonded to the sensor stage 3.
FIG. 7 is a perspective view describing the adhesion defects on the back surface of the sensor chip mounted on the sensor stage as shown in FIGS. 5(a) and 5(b). First, a sensor chip 4 is mounted on a sensor stage 3 and the adhesive between the sensor chip 4 and the sensor stage 3 is cured. Then, the sensor chip 4 is removed from the sensor stage 3 and the sticking state of the adhesive 7 is observed. As described in FIG. 7, the adhesive 7 localizes to traces 7a corresponding to the pins 3d (cf. FIGS. 4(a) and 4(b)) formed in the sensor stage 3. A region with almost no adhesive sticking thereto is observed in the central portion on the back surface of the sensor chip 4. Furthermore, it has been observed that the adhesive sticking to the central portion on the back surface of the sensor chip 4 is cured insufficiently, since the ultraviolet ray irradiated downward onto the sensor stage 3 in the UV-curing step does not well reach the central portion on the back surface of the sensor chip 4, causing carriers of an insufficient amount.
Once the adhesion defects as described above are caused in the manufacturing process, the sensor chip 4 will part unexpectedly from the sensor chip 3 by the impact exerted to the range finder module, causing incapability of range finding and such troubles. Thus, the reliability of the range finder module is impaired.
In view of the foregoing, it would be desirable to provide a semiconductor device module having an improved chip mount structure that facilitates preventing adhesion defects from causing when a sensor chip is bonded to a sensor stage with an adhesive and improving the reliability thereof.